In recent years, in connection with application of a high-order emission control relating to a diesel engine (hereinafter, refer simply to as an engine), it has been desired to mount an exhaust gas purification device removing air polluting substances in an exhaust gas to an agricultural machine and a construction machine having an engine mounted thereon. As the exhaust gas purification device, there has been known a diesel particulate filter (hereinafter, refer to as DPF) (refer to patent documents 1 and 2).
The DPF is structured such as to collect a particulate matter (hereinafter, refer to as PM) in the exhaust gas, and can recover its own PM collecting capacity (can self-renew) by burning and removing the PM, if a temperature of the exhaust gas is, for example, equal to or higher than 300° C. However, if a drive state of the engine is under low speed and low torque, and the temperature of the exhaust gas is lower, the PM is neither burned nor removed, but is deposited in the DPF (cannot be self-renewed). If a deposition amount of PM in the DPF exceeds a prescribed amount, a flow resistance in the DPF is increased and a reduction of an engine output is caused. Accordingly, a forced renewal of DPF has been often carried out by forcedly raising an output horsepower of the engine so as to raise the temperature of the exhaust gas.